Glass articles having the optical and mechanical characteristics required for use in information displays for consumer electronic devices such as cell phones, media players, computers, and televisions are presently in high demand. The performance attributes of such articles that are valued by manufacturers and users of electronic devices include low thickness, excellent optical quality, high resistance to surface damage from abrasion, and strength sufficient to withstand breakage or chipping from impacts or applied bending stresses, among others.
The resistance of glass articles to impact and flexural damage is generally determined by standardized flexural (bending) breaking stress as well as ball drop (impact) testing. As is known, the measured strengths of glass articles determined by such methods can be highly variable, depending in large part on the way in which the glass is prepared for testing and handled prior to testing. For consumer applications comprising glass articles, ball drop and bending strength test results indicating reduced variability in strength are as important to device designers as are results indicating high overall strength.
A number of methods for improving the mechanical properties of thin glass articles (e.g., having thicknesses of less than or equal to about 2 millimeters) for information displays are in current commercial use or under extensive development. Glass tempering methods for improving impact and flexural strength are well known and include, for example, chemical tempering (ion-exchange strengthening) methods that can develop high levels of compressive stress in the surfaces of such sheets.
As suggested above, although glass tempering methods can produce articles exhibiting very high resistance to impact and flexural damage, strength levels measured after tempering can be undesirably variable with some samples having high strength while others have significantly reduced strength. This strength variability has been attributed at least in part to the presence of handling-induced surface flaws in the articles prior to tempering. Among the measures being developed to address the problem of strength variability are glass etching treatments that can remove the flawed surfaces from such articles, either before or subsequent to tempering. In general, such treatments involve the use of fluoride-based chemical etchants, including such compounds as HF, NaF, and NH4HF2.
While glass surface etching methods have been shown to be effective for reducing strength variability in tempered glass articles, a number of problems attending the use of such methods, even beyond the high cost of etchants such as HF, have been identified. Most significant are the handling requirements for HF that present significant problems in large scale manufacturing environments. Further, fluoride etching produces significant quantities of by-product fluorides such as Na2SiF6, K2SiF6, CaF2, and the like that precipitate and collect to form sludge in etching tanks. Such sludge must be periodically removed and disposed of at considerable expense.
Also problematic is the aggressive nature of fluoride etchants. While a focused surface flaw removal or reduction treatment alone should be sufficient to improve glass strength, solutions containing fluoride ions rapidly etch the entire surfaces of the glass articles. As a consequence, even a brief exposure (e.g., two to five minutes) to an etching solution such as an HF/H2SO4 solution, which is effective to remove as little as 1.5 micrometers of surface glass from a flawed article, is equivalent to removing about 1500 pounds of glass, and can produce as much as 10,000-20,000 pounds of etched glass waste solution, for each 1 million square feet of glass being treated.
Other problems associated with the use of aggressive etchants include the possibility of non-uniform surface removal. Undesirable results of such removal can include reductions in article surface flatness or thickness that can interfere with accurate electronic device deposition, as well as reductions in glass optical quality caused by general surface hazing or localized changes in surface reflectivity. Extensive etching can also expose surface scratches previously present only as undetectable and harmless subsurface sheet damage.
The above-noted problems clearly indicate that there remains a need for improved glass article strengthening methods that can provide substantial improvements in article strength without increasing strength variability and without increasing the cost or reducing the efficiency of existing commercial glass strengthening processes.